Process and apparatus for producing sub-micron fibers, and nonwovens and articles containing same

ABSTRACT

A process and apparatus for producing sub-micron fibers, and more specifically a process and apparatus for effecting formation of sub-micron fibers by fibrillation of polymer films, and nonwoven materials and articles incorporating them.

TECHNICAL FIELD

The present invention generally relates to producing sub-micron fibers,and more specifically relates to a process and apparatus for effectingformation of sub-micron fibers by fibrillation of polymer films, andnonwoven materials and articles incorporating them.

BACKGROUND ART

Continuous and discontinuous filament spinning technologies are known inart, and are commonly referred to as spunmelt technologies. Spunmelttechnologies include both the meltblown or spunbond processes. Aspunbond process involves supplying a molten polymer, which is thenextruded under pressure through a large number of orifices in a plateknown as a spinneret or die. The resulting continuous filaments arequenched and drawn by any of a number of methods, such as slot drawsystems, attenuator guns, or Godet rolls. The continuous filaments arecollected as a loose web upon a moving foraminous surface, such as awire mesh conveyor belt. When more than one spinneret is used in linefor the purpose of forming a multi-layered fabric, the subsequent websare collected upon the uppermost surface of the previously formed web.

The melt-blown process is related to means of the spunbond process forforming a layer of a nonwoven fabric, wherein, a molten polymer isextruded under pressure through orifices in a spinneret or die. Highvelocity gas impinges upon and attenuates the filaments as they exit thedie. The energy of this step is such that the formed filaments aregreatly reduced in diameter and are fractured so that microfibers ofindeterminate length are produced. This differs from the spunbondprocess whereby the continuity of the filaments is preserved.

Spunmelt equipment manufacturing companies, such as Reifenhäuser, AsonNeumag, Nordson, and Accurate Products have engineered numerousmeltblown and/or spunbond manufacturing models that offer a variety ofdesirable attributes, such as increased polymer throughputs, bettermanagement of process air flow or polymer distribution, and improvedcontrol of filament deviations, to name a few. U.S. Pat. Nos. 4,708,619;4,813,864; 4,820,142; 4,838,774; 5,087,186; 6,427,745; and 6,565,344,all of which are incorporated herein by reference, disclose examples ofmanufacturing equipment for the processing of spunbond or meltblownmaterials.

There is an increasing demand for articles produced from nonwovencontaining sub-micron fibers. The diameters of sub-micron fibers aregenerally understood to be less than about 1000 nanometer (i.e., onemicron). The sub-micron fibers webs are desired due to their highsurface area, low pore size, and other characteristics. The sub-micronfibers can be produced by a variety of methods and from a variety ofmaterials. Although several methods have been used, there are drawbacksto each of the methods and producing cost effective sub-micron fibershas been difficult. Conventional spunmelt equipment arrangements can notprovide high quality, low defect fibers and webs that are predominantlymicrofine including submicron diameter fibers with narrow fiber sizedistributions.

Methods of producing sub-micron fibers include a class of methodsdescribed by melt fibrillation. Non limiting examples of meltfibrillation methods include melt blowing, melt fiber bursting, and meltfilm fibrillation. Methods of producing sub-micron fibers, not frommelts, are film fibrillation, electro-spinning, and solution spinning.Other methods of producing sub-micron fibers include spinning a largerdiameter bi-component fiber in an islands-in-the-sea, segmented pie, orother configuration where the fiber is then further processed so thatsub-micron fibers result.

Melt fibrillation is a general class of making fibers defined in thatone or more polymers are molten and extruded into many possibleconfigurations (e.g. co-extrusion, homogeneous or bicomponent films orfilaments) and then fibrillated or fiberized into filaments.

Melt film fibrillation is another method to produce fibers. A melt filmis produced from the melt and then a fluid is used to form fibers fromthe melt film. Two examples of this method include Torobin's U.S. Pat.Nos. 6,315,806; 5,183,670; and 4,536,361; and Reneker's U.S. Pat. Nos.6,382,526, 6,520,425 and 6,695,992, assigned to the University of Akron.

Electrospinning is a commonly used method of producing sub-micronfibers. In one manifestation of this method, a polymer is dissolved in asolvent and placed in a chamber sealed at one end with a small openingin a necked down portion at the other end. A high voltage potential isthen applied between the polymer solution and a collector near the openend of the chamber. The production rates of this process are very slowand fibers are typically produced in small quantities. Another spinningtechnique for producing sub-micron fibers is solution or flash spinningwhich utilizes a solvent.

One of the ways to achieve high melt shear for a melt film fibrillationprocess is through high-speed gas (i.e., close to sonic or supersonicgas velocities). To obtain supersonic or transonic (close to sonic)velocities, flow typically needs to converge to a throat (narrowest partof the nozzle where the velocity reaches sonic levels) and then expandsin a diverging section. Adiabatic nozzles (no heat gained or lostthrough the boundaries of the nozzle system) meeting these generalcriteria are known in the art, and include so-called Laval nozzles. Useof Laval type nozzles in fiber formation are disclosed, e.g., in U.S.Pat. Appln. Publ. No. 2004/0099981 A1, and U.S. Pat. Nos. 5,075,161 and5,260,003. These methods utilize Laval nozzles to speed up the gasvelocities to sonic and/or supersonic range. When polymer melt isexposed to such high gas velocities, it bursts into multiplicity of finefibers. They generally use concentric input and channeling of gas andpolymer melt in the discharge nozzles, which can be non-optimal fromstandpoints of equipment lay-out complexity and equipment maintenance,etc. However, other nozzle configurations, such as non-concentric(non-annular) layouts of nozzles pose challenges of their own. Forinstance, in a fiber or filament nozzle system where polymer melt andgas introduction proceed from separate side-by-side units, a problemtends to arise when the fiberization gas flows between a side with asurface or wall that is heated to a high temperature (e.g., due topolymer melt flow introduction from that side) and a surface or wall ofan opposing side (for example, an gas introduction side) that is at alower temperature than the polymer melt side. In such a scenario, gasflow tends to become unstable in the diverging section in prior nozzleconfigurations such as with a Laval nozzle. This leads to problems oflack of polymer shear, polymer back-flow or build-up into the gas sideof the gas passage, and subsequently an unevenly varying excessive anddiminishing polymer flow and fiberization. After sufficient melt buildup occurs upstream into the gas side, polymer melt separates and istypically blown out as a “shot,” since the melt locally cools down andcan no longer form fibers due to insufficient shearing of polymer. Whenthe polymer flow starves as the other end of those variation, theshearing is excessive leading to undesired dust. New advances have beenneeded to allow production of consistently high quality sub-micronfibers for disposable articles in a more efficient manner atcommercially-significant output levels.

SUMMARY

The present invention is directed to high quality, low defect sub-micronfibers and nonwovens incorporating the sub-micron fibers that areproduced in a unique single step, melt film fibrillation, highthroughout process, and a nozzle device used for this purpose. Nonwovenproducts are attained that contain high quality microfiber content,which in one aspect exceeds 99% sub-micron fiber content, at commercialscale throughputs. Increased polymer shear and reduction of polymerback-flow or build-up problems otherwise leading to undesired fiberdefects, such as shot development within a nozzle system, are alsoachieved by the present invention. With the present invention, highquality, microfibrous nonwoven products having improved barrierproperties, softness, absorbency, opacity and/or high surface area areprovided that are suitable for a large variety of industrial andconsumer care fibrous products.

A process for making a nonwoven web has been found for producing highquality, high output sub-micron fiber product by providing a pressurizedgas stream flowing within a gas passage confined between first andsecond opposing walls which define respective upstream converging anddownstream diverging wall surfaces into which polymer melt is introducedto provide an extruded polymer film on a heated wall surface that isimpinged by the gas stream flowing within the gas passage, effective tofibrillate the polymer film into sub-micron diameter fibers.“Converging” means that the cross-sectional area decreases in thedirection of gas flow; and “diverging” means that the cross-sectionalarea increases in the direction of gas flow. In one embodiment, the gaspassage comprises a first, upstream section into which the gas entersfrom a supply end, a transition region, and a second, downstream sectionin which the gas flows to an exit end, wherein the transition regionfluidly connects the first section to the second section, and the gaspassage ends at the exit end of the second section. In a particularembodiment, the first section of the gas passage has a monotonicallydecreasing cross-sectional area from the supply end to the transitionregion, and the second section of the gas passage has a monotonicallyincreasing cross-sectional area from the transition region to the exitend of the second section. At least one flowing polymer fluid stream istransmitted through at least one bounded polymer passage which ends inat least one opening in at least one of the opposing heated walls.Polymer is heated sufficiently in transit to make and keep it flowableuntil introduced into the gas passage. Each polymer fluid streamextrudes in the form of a film from each opening. Each extruded polymerfilm joins with the gas stream and the polymer film is fibrillated toform fibers comprising sub-micron fibers exiting from the exit end ofthe second section of the gas passage. For purposes herein,“monotonically decreasing cross-sectional area” means “strictlydecreasing cross-sectional area” from the upper (inlet) end to the lowerend of the upstream nozzle section, and “monotonically increasingcross-sectional area” means “strictly increasing cross-sectional area”from the upper end to the exit end of the downstream section of thenozzle.

Although not desiring to be bound to any theory, it is thought that theintroduction of heated polymer as a film on a heated support wall whichin part defines the gas passage within the nozzle as described hereinmakes it possible to maintain and control gas flow uniformity in anenhanced manner such that the fibrillated fiber product has improvedsize distribution that is weighted towards or is even exclusively in thesub-micron fiber size range.

In a particular embodiment, each extruded polymer film joins with thegas stream in the second section of the gas passage. The introduction ofthe polymer melt in the second section of the nozzle system on a heateddiverging support wall has been found to especially facilitateproduction of high quality, high content sub-micron fibers and resultingwebs at commercial throughputs. In a further embodiment, the locationwhere the extruded polymer film joins with the gas in the second,downstream section in order to produce the best quality fibers and webdepends on the type of gas, the nozzle geometry, including angles andtransitions, and the pressure of the gas, and is preferably located inthe upper half of the second section such as for low gas pressureconditions, and is preferably located in the lower, downstream half ofthe second section such as for high gas pressure conditions. In aparticular embodiment, only one polymer film forms on at least one ofthe opposite heated walls, the gas pressure exceeds about 10 psi, andeach polymer passage opening from which polymer film extrudes is locatedin a second, downstream half of the second section between thetransition region and the exit end of the second section. It has beenfound that the second half of the downstream second section can providean optimal gas velocity region where melt film fibrillation isaccomplished very efficiently, yielding higher quality microfiberproduct.

As another advantage of the present invention, increased sub-micronfiber output is obtained with lower gas demand. Lowered gas demand makesit possible to reduce energy consumption and/or use smaller scale unitoperations to still provide commercially significant sub-micron fiberoutput levels. In one embodiment, the gas stream and polymer fluidstream are introduced into the second section at a gas stream/polymerfluid stream mass flow rate ratio less than about 40:1, particularlyless than about 30:1; more particularly less than about 15:1. The gasstream to polymer fluid stream mass flow ratio is calculated as kilogramper hour per meter of gas stream through the gas passage to kilogram perhour per meter of polymer fluid stream through all the polymer openingsin the second section of the gas passage.

In more particular embodiments, each polymer passage opening may be aslit with a hydraulic diameter defined as four times cross-sectionalarea of the polymer passage opening divided by inner perimeter of thepolymer passage opening, said hydraulic diameter of each polymer passageopening ranging from about 0.001 inch to about 0.100 inch. The polymerfilm generally has a polymer film thickness not exceeding the hydraulicdiameter of the polymer passage opening. The polymer fluid may expandupon exiting the polymer passage opening, for example, due to die swellphenomenon without being limited by theory. However, the polymer fluidfilm thickness almost instantaneously becomes lesser than or equal tothe hydraulic diameter of the polymer passage opening.

In characterizing the geometry of the wall-defined gas passage of thenozzle of the present invention, a first bisecting surface, defined asan angular bisector of the angle between the first and second walls inthe first section, geometrically divides the first section into twohalves with about equal volumes, and a second bisecting surface, definedas an angular bisector of the angle between the first and second wallsin the second section, geometrically divides the second section into twohalves with about equal volumes. The bisecting surface may be planar orcurvilinear, depending on the embodiment of the present invention, aswill be more apparent from the detailed descriptions herein. In ageneral embodiment, the bisection angle of the first and the secondwalls with respect to the first bisecting surface ranges from about 15to about 40 degrees in the first section, and the bisection angle of thefirst and the second walls with respect to the second bisecting surfaceranges from about 2 to about 20 degrees in the second section of the gaspassage.

The opposing walls of nozzle where polymer is introduced into the gaspassage may be operated such that they are thermally similar ordifferent. In one embodiment, the first and second walls of the gaspassage are heated to approximately the same temperature to providesymmetric thermal states with respect to the first and the secondbisecting surfaces. In an alternative embodiment, one of the opposingwalls may be a hot wall while the other wall is a cold wall, whereintemperature of the hot wall is at least higher than the cold wall, suchas at least 50° C. higher, and only the hot wall has at least onepolymer fluid passage opening. In this configuration, the microfibersmay be produced in a hot melt/“cold” gas (e.g., unheated air)fibrillation environment that reduces process complexity and costs. Inthis embodiment, the hot wall in the second section diverges away fromthe first bisecting surface at an angle that ranges from about 1 degreeto 20 degrees, and the cold wall in the second section converges towardsthe first bisecting surface at an angle that ranges from about 0.1degree to about 15 degrees. The ratio between the diverging angle of thehot wall relative to the first bisecting surface and the convergingangle of the cold wall relative to the first bisecting surface may rangefrom about 1:1 to about 500:1. The angle between centerline of eachpolymer passage and the wall containing the corresponding polymerpassage opening may range from about 10 degrees to about 100 degrees.The polymer film extruding from each polymer passage opening may flowwith the gas flow along a polymer fiberization surface, which has anorientation angle with respect to the first bisecting surface rangingfrom about 90 degrees measured in clockwise direction to about 45degrees measured in counterclockwise direction. The length of thepolymer fiberization surface corresponding to each polymer passageopening may be less than about one thousand times the hydraulic diameterof the corresponding polymer passage opening.

In another embodiment, the first and second walls of the gas passage aresmoothly curved such that the curvature of the opposing walls in thefirst section smoothly transitions without any sharp edges into thecurvature of the opposing walls in the second section in the region ofsmallest cross-section area of the gas passage. The opposing walls inthe second section of the gas passage may be curved such that the hotwall has a convex shape that curves away from the second bisectingsurface and the cold wall has a concave shape that curves towards thesecond bisecting surface as viewed from within the second section in thegas passage. The ratio of the radius of curvature of the hot wall to theradius of curvature of the cold wall in the second section of the gaspassage ranges from about 1:10,000 to about 100:1. The gas stream isintroduced into the gas passage at a mass flow rate ranging from about150 kilogram per hour per meter to about 3500 kilogram per hour permeter.

The nozzle used in the processes described herein for making nonwovenwebs comprising sub-micron fibers represents another embodiment of thepresent invention. The inventive nozzle device is not limited to anyparticular type of polymer material or fibrillating gas and allows forthe polymer to be independently selected for a specific application fromamongst a wide variety of polymeric materials. Particularly thefibrillating gas is a gaseous material such as air, nitrogen, steam,etc. The gas may be used as a single type thereof or as combinations ofdifferent gases. Additionally suitable gases may include reactive gasesor gases with reactive components, or combinations thereof. Inembodiments, the gas generally may be inert to the nozzle wallmaterials. For purposes herein, the terms “nozzle system” and “nozzle”are used interchangeably.

The high quality microfibers provided by the present invention areprovided within narrow fiber size distributions with minimal fiberdefects. The raw nonwoven web product materials directly collected fromthe process of the present invention generally may comprise more than35%, particularly more than 75%, and more particularly more than 99%sub-micron fibers. The standard deviation of fiber diameter distributiongenerally may be less than about 0.5 micron, particularly less thanabout 0.3 micron. The present invention also may be used in productionof microfibers in the range of meltblown fibers. The present inventioncan be implemented on a wide variety polymer materials. The fibers maybe comprised of a polymer, e.g., selected from amongst polyolefins,polyesters, polyamides, biodegradable polymers, polyurethanes,polystyrenes, alkyd resins, poly-hydroxyalkanoic acids, adhesives andother compounds capable of making fibers, and combinations thereof. Thenonwoven web may be used in a wide variety of articles by itself or incombination with other materials. The nonwoven web may be used, forexample, in filters, medical apparel, medical cleaning wipes, housewrapconstruction materials, bandages, protective clothing, batteryseparators, catalyst carrier, diapers, training pants, adultincontinence pads, catamenials products such as feminine care pads andpantiliners, tampons, personal cleansing articles, personal carearticles, and personal care wipes such as baby wipes, facial wipes, bodywipes and feminine wipes, and combinations thereof.

Other features and advantages of the present invention will becomereadily apparent from the following detailed description, theaccompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an enlarged generalized sectional view of a nozzle system ofthe present invention for forming sub-micron fibers.

FIG. 2 is a sectional view taken at Section 120 shown in FIG. 12 of anillustrative embodiment of a nozzle system with a single polymer meltintroduction passage according to an embodiment of the present inventionfor forming sub-micron fibers.

FIG. 3 is a sectional view of an illustrative embodiment of a nozzlesystem with multiple polymer melt introduction passages according toanother embodiment of the present invention for forming sub-micronfibers.

FIG. 4 is a sectional view of an illustrative embodiment of a nozzlesystem including a divergent wall on the polymer introduction side andan opposing convergent wall in the downstream nozzle section accordingto another embodiment of the present invention for forming sub-micronfibers.

FIG. 5 is a sectional view of an illustrative embodiment of a nozzlesystem with curved wall surfaces according to another embodiment of thepresent invention for forming sub-micron fibers.

FIG. 6 is a sectional view of an illustrative embodiment of a nozzlesystem including a defined impingement surface according to anotherembodiment of the present invention for forming sub-micron fibers.

FIG. 7 is an enlarged sectional view of a downstream portion of thenozzle system according to FIG. 6.

FIG. 8 is a sectional view of an illustrative embodiment of a nozzlesystem with a curvilinear bisecting surface for the gas passage in theupstream and downstream sections according to another embodiment of thepresent invention for forming sub-micron fibers.

FIG. 9 is a sectional view of an alternative embodiment of the nozzlesystem of FIG. 8.

FIG. 10 is a sectional view of an illustrative embodiment of a nozzlesystem of another embodiment of the present invention for formingsub-micron fibers.

FIG. 11 is a sectional view of an alternative embodiment of the nozzlesystem of FIG. 10.

FIG. 12 is an isometric view of the nozzle system of FIG. 1.

FIG. 13 is a plan view of the top side of the nozzle of FIG. 12.

FIG. 14 is a plan view of the bottom side of the nozzle of FIG. 12.

FIG. 15 is an SEM microphotograph (500×) of microfibers with shot.

FIG. 16 is an SEM microphotograph (500×) of microfibers with very littleor no shot.

The features depicted in the figures are not necessarily drawn to scale.Similarly numbered elements in different figures represent similarcomponents unless indicated otherwise.

DETAILED DESCRIPTION

While the present invention is susceptible of embodiment in variousforms, there is shown in the drawings and will hereinafter be describeda presently preferred embodiment of the invention, with theunderstanding that the present disclosure is to be considered as anexemplification of the invention, and is not intended to limit theinvention to the specific embodiment illustrated.

Referring to FIG. 1, a general nozzle system 800 is depicted for makingfibers, and by way of example, an annular, axisymmetric system is shown.Gas is pressurized in element 700, from which gas stream 3 is suppliedand enters a first nozzle section 8 having a generally converginggeometry and flows towards and through a transition region 9, and thenthe gas stream enters into and expands in a second nozzle section 10having generally diverging geometry before leaving the nozzle systemthrough the exit plane 101 into the atmospheric environment 900. Thetransition region 9 represents a narrowed throat section of the nozzlein which the upstream converging section is transitioned into thedownstream diverging section. The transition region or throat includesthe smallest cross-sectional area of the nozzle. Polymer melt is fedfrom a polymer extrusion body 801 or other molten polymer source to thegas passage 802 or interior of the nozzle 800. As indicated by thenon-limiting dashed lines 804-805 depicted in the figure, polymer meltcan be introduced anywhere in the nozzle 800 provided that a polymerfilm is provided on an inner heated wall surface 803 of the nozzle 800that is impinged by the gas stream 3 flowing within the gas passage 802,effective to fibrillate the polymer film into sub-micron diameterfibers.

Referring to FIG. 2, a nozzle system 1 is illustrated for makingmicrofibers, and particularly nonwoven products comprising sub-microndiameter fibers in web or mat form, according to embodiments of thepresent invention. The nozzle system 1 shown in FIG. 2 depicts asymmetric gas passage 4, by way of example. A polymer fluid stream 2 isintroduced into nozzle system 1 along a curved, straight or othergeometrically-suitable polymer passageway 200. An imaginary bisectingaxis or plane 7 geometrically bisects the space between the opposingwalls 51 and 61 in the first section 8, and also the opposing walls 5and 6 in second section 10. As can be appreciated, if the opposing wallsare rectilinear (i.e., generally planar) sloped surfaces, then thebisector 7 is a plane, while if the opposing walls curve to provide acontinuous concentric surface, then the bisector 7 is a longitudinalaxis. If they are opposing sloped generally planar walls, thenspace-apart upright forward and rearward sidewalls are also providedthat connect the opposing sloped walls (5, 6, and 51, 61), effective tocomplete the enclosure of gas passage 4 in an fluid-tight manner. Forexample, a rear side wall 43 is indicated in FIG. 2. The correspondingforward side wall is similar but not shown in FIG. 2 to simplify thisillustration.

With further reference to FIG. 12, the same nozzle system 1 isillustrated in a manner showing rear side wall 43 and a forward sidewall 44. The walls 43 and 44 are joined in a fluid-tight manner toopposite ends 1210/1211 and 1221/1222 of die components 121 and 122,respectively, which include the above-described opposing walls thatdefine the gas passage extending through the first and second sectionsof the nozzle. The die and end wall components can be made of material,e.g., polymer, metal, ceramic, etc., that can be shaped, e.g., bymolding, casting, machining, etc. into the appropriate shapes, and arecomponents which can tolerate the microfiber production processconditions, such as described herein. In FIG. 12, the location and shapeof the stacked dual-funnel shape defined by the opposing walls is tracedwith imaginary lines at the end walls 43 and 44 to facilitate theillustration, although it will be appreciated that the end walls 43 and44 close off the gas passage 4 and the rearward and forward oppositeends of the nozzle. As illustrated in FIG. 12, the upper nozzle inlet 41is space defined between upper edges 510 and 610 of dies 121 and 122,respectively. The nozzle exit 42 is space defined between lower edges500 and 600 of dies 121 and 122, respectively.

FIGS. 13 and 14 show a nozzle inlet cross-sectional area 1001 (indicatedby cross-sectioned area in FIG. 13 defined between edges 610 and 510)and nozzle exit cross-sectional area 1002 (indicated by cross-sectionedarea in FIG. 14 defined between edges 500 and 600) of the nozzle inletopening 41 and exit opening 42, respectively, defined by the diecomponents 121 and 122. Also shown in FIG. 13 is an intermediatecross-sectional area 1003 defined between opposing wall locations 1004and 1005 (indicated by hatched lines) located between nozzle inlet 41and the transition region 9 of the nozzle. Also shown in FIG. 14 is anintermediate cross-sectional area 1006 defined between opposing walllocations 1007 and 1008 (indicated by hatched lines) located between thetransition region 9 and the nozzle exit 42 of the nozzle. In FIGS. 13and 14, die edges defining the gas passage that are not visible in thegiven view have locations indicated generally by dashed lines.

As illustrated, the cross-sectional area of the first section 8 isdecreasing, preferably at least substantially continuously, in thedownstream direction between the inlet 41 through intermediate area 1003and further until reaching the transition region 9. The cross-sectionalarea of the second section 10 is increasing, preferably at leastsubstantially continuously, in the downstream direction between thetransition region 9 through intermediate area 1006 and further untilreaching the exit opening 42 of the nozzle. In a particular embodiment,the first section 8 of the gas passage 4 has a monotonically decreasingcross-sectional area 1001 from the supply end 41 to the lower dischargeend 410 of the first section 8, i.e., the beginning of the transitionregion 9, and the second section 10 of the gas passage 4 has amonotonically increasing cross-sectional area 1002 from the entrance 420or beginning of the second section 10 (i.e., the lower end of transitionregion 9) to the exit end 42 of the second section 10. These criteriaalso are used in the following additional embodiments of the presentinvention.

Referring now to FIG. 3, multiple polymer fluid streams 2 also may beintroduced concurrently from multiple corresponding polymer introductionpassages 200 a-d that feed into gas passage 4. The number of polymerfluid streams is not limited other than by practical constraints of agiven nozzle set-up. A pressurized gas stream 3 is introduced within gaspassage 4 and flows in a direction 30 from upstream nozzle section 8through transition region 9 to downstream nozzle section 10. The firstand second opposing walls 5, 6 comprise first section 8 and a secondsection 10. The first and second opposing walls 5, 6 converge in thefirst section 8 towards the transition region or throat section 9, whichis the narrowest cross-section of the gas passage 4, as measured in adirection normal to the imaginary bisecting plane 7 between nozzle inlet41 and nozzle exit 42. Thus the throat section 9 connects the firstsection 8 to the second section 10 and conduits gas from one section tothe other. At least one of the opposing walls 5 or 6 diverges from plane7 in the second section 10. In this illustration, the first section 8has a continuously, progressively decreasing cross-sectional area 65measured between the opposing walls 51 and 61 in the gas flow direction30 from inlet 41 to throat 9. The second section 10 has continuously,progressively increasing cross-sectional area 66 measured betweenopposing walls 5 and 6 in the gas flow direction 30 from throat 9 toexit 42. These cross-sectional areas 65 and 66 are measured normal tothe plane 7 between the opposing walls (i.e., 51, 61 or 5, 6, asapplicable) in the direction of the gas flow 3. The polymer fluidstreams 2 leave the polymer introduction passage 200 or passages 200 a-dat the polymer passage openings 20 and flow out on the opposing walls 5and 6, and are combined with the high velocity gas stream 3, preferablyin the second section 10 of the gas passage 4, to form films 11, i.e.,polymer melt exiting the polymer passage openings spreads into rivuletsin the form of a film or otherwise uniformly spread out polymer fluid.The film or films are fibrillated to form fibers 12 comprisingsub-micron diameter fibers that are collected below the nozzle system 1as a fibrous web or mat material 13. The amount of fibrillation of films11 from polymer fluid streams 2 may be different depending on thelocation of passages 200 a-d in the gas passage 4. In a non-limitingembodiment, fibrillated polymer melt in the form of continuous ordiscrete melt filaments or melt particles, e.g., due to excessiveshearing of films 11 from passages 200 b and 200 c, may combine withfibrillated polymer melt films 11 from passages 200 a and 200 d,respectively. In such embodiment, if polymer fluid streams 200 b and 200c are respectively of dissimilar polymer types from polymer fluidstreams 200 a and 200 d, the fibrous web material may comprise ofmulti-component fibers, or more specifically bicomponent fibers. Theintroduction of the polymer melt in the second section of the nozzlesystem on a heated diverging support wall has been found to especiallyfacilitate production of high quality, high content sub-micron fibersand resulting webs at commercial throughputs. The polymer passageopenings 20 into the gas passage 4 can have oval, round, rectangular orother geometric cross-sections. There may be single or multiple polymerpassage openings into the gas passage on either opposing surface/wall.The single or multiple openings in the preferred embodiment are on thehotter sides, the polymer melt-side (e.g., see FIG. 6 described ingreater detail below).

Referring again to FIG. 3, it has been found that the location of thepolymer passage opening 20 for making high quality fibers depends on thetype of gas used, the geometry of the nozzle sections, and the pressureof the gas. In one preferred embodiment, the pressure of the enteringgas is relatively low, less than about 10 psi, and the extruded polymerfilm joins with the gas in the upper half (50%) of the second,downstream section 10, indicated as section 101, in which polymer meltis extruded from gas passage 200 c. It has been found in this case thatthe upper half of the downstream second section 10 can provide anoptimal gas velocity region where melt film fibrillation is accomplishedvery efficiently, yielding higher quality microfiber product.Consequently, it is a preferred embodiment in the case of pressure ofthe entering gas, exceeding about 10 to 15 psi, that the extrudedpolymer film joins with the gas in the lower, downstream half (50%) ofthe second, downstream section 10, indicated as remainder after section101, in which polymer melt is extruded from gas passage 200 a. As thegas pressure is increased, the preferred location for joining the gasand polymer streams moves downstream, i.e. from passage 200 c to passage200 b to passage 200 d and to passage 200 a for the highest range of gaspressures.

The fibrous web 13, such as shown in FIGS. 2-3, may be comprised of apile of loose fibers or alternatively a self-supporting unitary web offibers, depending on process conditions such as temperature, collectordistance 100, and so forth. The fibers may also be deposited on a movingsubstrate web to form an additional layer. Collection of the fibersdischarged from the nozzle system 1 may be done, e.g., onto a belt orsubstrate 300 aided by a vacuum underneath the belt or substrate orother means to keep the fibers deposited on the belt or substrate untilfurther processing. The fiber collecting structure may be, e.g., a meshor belt across which a vacuum pulls the fibers onto the structure. Itcan also comprise a preformed fibrous web. It is obvious to thoseskilled in the art that the nozzle system can be of essentially slotdesign or of annular design with minor modifications. Although FIGS. 2and 3 show a nozzle with essentially flat and symmetric geometry, thatis shown only for simple non-limiting illustration purposes.

FIG. 4 shows more details of a nozzle system 1, and in this illustrationshows a system with a colder apparatus side that is bounded by thecolder first opposing wall 5, and a hotter apparatus side containing thepolymer melt components that is bounded by the hotter second opposingwall 6. An axis or plane 7 geometrically bisects the space between theopposing walls 5 and 6 in the first section 8, thus also defining thecontraction bisection or half-angles α. Preferably, the bisection angleα is between 0.5 and 89.5 degrees, more preferably between 1 and 45degrees, and most preferably between 15 and 40 degrees. In one of theembodiments the bisection α is about 30 degrees. The opposing wall 6diverges from the axis or plane 7 in the second section 10, while theoverall cross-sectional area 66 of the gas passage 4 in the secondsection 10, as measured in a direction normal to the direction 30 of gasflow, actually still increases, allowing the gas to expand after thethroat section 9. The opposing colder wall 5 is generally convergingwith respect to the axis or plane 7 at an angle θ. The angle β ismeasured from the hotter opposing wall 6 to the bisecting axis or plane7, and the angle θ is measured from the axis or plane 7 to the colderopposing wall 5. Therefore, the angle θ is zero if the opposing wall 5is parallel to the bisecting axis or plane 7, and negative if it isconverging, and positive if it is diverging. The opposing wall 6 has adiverging angle β that should generally be from about 1 degree to lessthan about 90 degrees relative to the axis or plane 7, or preferablyfrom about 2 to less than about 20 degrees. In one embodiment thisdiverging angle β is approximately 15 degrees. The opposing wall 5 has adiverging angle that should be less than about +45 degrees relative tothe axis or plane 7, but preferably has a converging angle that is fromabout −45 degrees to about zero degree. The sum of β and θ shouldpreferably be from about 0.1 degree to about 30 degrees. Polymer isdirected through the polymer introduction passages 200 and exiting intothe gas passage 4 through one or more polymer passage openings 20 andflowing in the form of film, rivulets, or hollow tubes, and preferablyfilm, onto the polymer fiberization surfaces 63, also referred to asimpingement surfaces. The polymer melt passages 200 can be at variousangles γ with respect to the hotter opposing wall 6, anywhere fromperpendicular (90 degrees) to almost parallel (co-flowing) to the hotteropposing wall surface 6 (about 5 degrees) or counterflowing at about 170degrees, but preferably from 10 to 100 degrees. The polymer fiberizationsurface 63 is present below (in the direction of gas flow) each polymerpassage opening 20 and has a smooth continuation of the second opposingwall 6 from before the introduction of the polymer melt 2, and on whichthe polymer melt flows out before and during fiberization by the gasstream 4. If the polymer fiberization surface 63 is not a smoothcontinuation of the second opposing hotter wall 6, then it can be at anangle φ that is measured between the polymer fiberization surface beloweach polymer passage opening in the direction of polymer flow and therespective polymer passage 2, and is less than about 180 degrees. Withan angle φ of (90-γ) degrees or lesser relative to polymer passage (inan embodiment with γ less than 90 degrees), the polymer fiberizationsurface 63 would essentially become zero. Alternatively, the orientationof the polymer fiberization surface may be measured relative to thebisecting plane or axis 7. When measured relative to the bisecting planeor axis 7, the polymer fiberization surface orientation angle rangesfrom about 90 degrees measured in clockwise direction to about 45degrees measured in counterclockwise direction. The length “δ” of thepolymer fiberization surface 63 is critical for a good fiberizationprocess with few fiber defects such as shot etc., and should be lessthan about one thousand times the hydraulic diameter of the respectivepolymer passage opening 20, but preferably less than one hundred timesthe hydraulic diameter of the respective polymer passage opening 20. Areason for this configuration example is that the opposing wall 6 isheated to keep the pressurized polymer stream 2 molten and flowing. Theheated wall 6 in the second section 10 of the gas passage 4 has adiverging angle β of less than 45 degrees from the central, bisectingaxis 7, preferably from about 1 to 20 degrees. The colder, unheated wall5 opposite to the heated wall 6 in the second section 10 of the gaspassage 4 may have a converging angle θ of less than 30 degrees from thecentral axis 7, particularly from about 0.1 to 15 degrees. The ratio,β/θ, of the diverging angle β to the converging angle θ of the opposingwalls 6 and 5, respectively, in the second section 10 of the gas passage4 ranges between 1:1 to 500:1. The cross-sectional inner geometry of thethroat 9 may be, e.g., rectangular shaped if the opposing walls aresloped (e.g., see FIG. 2), or alternatively may be rounded if curvedopposing walls are used that form a continuous curved inner boundarydefining the gas passage. In another embodiment, the gas passage 4comprises an annular cross-sectional space located between walls 5 and6. The walls in FIG. 4 are shown as sloped, flat and planar, however thewalls in the various sections may be curved. This may be done for walls5 and 6 in either the first, or the second or the throat sections, or acombination of them. A particular, non-limiting embodiment is presentedin the following description.

Referring to FIG. 5, in another embodiment of the nozzle system 1, theopposing walls 6 and 5 in the second section 10 of the gas passage 4 arecurved. The curved portion may have a vertical dimension 102 of about0.004 inch to about 2 inch, and the melt passage 200 may have ahydraulic diameter of about 0.001 inch to 0.100 inch. The ratio, r₁/r₂,of the radius of curvature r₁ of the hotter wall 6 to the radius ofcurvature r₂ of the colder wall in the second section 10 of the gaspassage 4 may range between 1:10,000 to about 100:1.

Referring to FIG. 6, in another embodiment of the nozzle system 1, thegeometry of the heated wall 6 is similar to that described in connectionwith FIG. 5 above. However, in this embodiment, the throat section 9 ofthe gas passage 4 has greater length in comparison to the embodiments ofFIGS. 2-4. The wall 5 opposite to the heated wall 6 converges towardsthe bisecting axis or plane 7 at an angle θ typically ranging from about0.1 degree to about 15 degrees. The heated wall 6 diverges from theplane 7 at an angle β typically ranging from about 1 to about 20degrees. As best seen in FIG. 7, the polymer fluid stream 2 enters thesecond section 10 of the gas passage 4 through one or a multiplicity ofopenings 20, and is directed at an angle γ that can range from about 10to 170 degrees, but is typically ranging from about 30 to about 150degrees, particularly about 60 to about 95 degrees, relative to thesecond, hotter wall 6. As shown, the heated wall 16 comprises a tipportion 65 immediately below a polymer introduction passage 2. Theintegral tip portion 65 has a polymer fiberization surface length lessthan about 0.050 inch, more preferably less than about 0.010 inch. Thecurved portion 64 is located immediately below the tip portion 65 curvesaway from the central axis 7 in the downstream direction 30, and thewall 5 opposite to the heated wall converges towards plane 7 along wallportion 151 thereof and at a location laterally spaced from and adjacentthe lower end of tip portion 152 is bent back to form the angle 90+θwith plane 7. The curved portion 64 defines the tip portion 65 at itsupper end and also helps prevent gas disruption within the gas passage 4near the polymer introduction passage 2. The tip portion 65 enhancessub-micron fiber formation. In this illustration, the tip portion 65 hasa side 63 facing the gas passage 4 that may be substantially flat anddefines the polymer fiberization length δ. When angles γ, β and θ arenot in the ranges prescribed herein, the process may be adverselyimpacted. For instance, it can negatively impact fiberization, fibersizes, and increase undesirable shot formation. As a non-limitingillustration, the tip portion 65 may have a vertical length orimpingement length δ of approximately 0.005-0.050 inch, the curvedportion 64 may have a vertical dimension of about 0.040-0.100 inch orgreater, and the melt passage 2 may have a hydraulic diameter of about0.001 to about 0.010 inch, and preferably from 0.002 to about 0.008inch.

Referring to FIGS. 8-9, the first and second walls (5, 6, and 51, 61) ofthe gas passage 4 of these alternate configurations of nozzle 1 aresmoothly curved such that the curvature of the opposing walls 5, 6 inthe first section 8 smoothly transitions in the transition region 9 ofthe gas passage 4 without any sharp edges into the curvature of theopposing walls 51, 61 in the second section 10 where polymer 2 isintroduced in this illustration. The opposing walls in the secondsection 10 of the gas passage 4 are curved such that the hot wall 6 hasa convex shape that curves away from the curvilinear bisecting surface 7and the cold wall 5 has a concave shape that curves towards thecurvilinear bisecting surface 7 as viewed from within the second section10 in the gas passage 4. The ratio of the radius of curvature of the hotwall 6 to the radius of curvature of the cold wall 5 in the secondsection 10 of the gas passage 4 may range from about 1:10,000 to about100:1, particularly about 1:4 to about 1:1, and more particularly about1:2 to about 1:1. The radii of curvature of the first and the secondwalls 51, 61 in the first second section 8 may range from about 1% toabout 1000% of the length of the first and the second walls 5, 6 in thesecond section 10 of the gas passage 4. Each polymer introductionopening 20 particularly may be located in the hot wall 6 in the secondsection 10. Each polymer opening 20 also may be located between about20% to about 80% of the curvilinear length of the hot wall 6 in thesecond section 10 of the gas passage 4. The polymer film can be extrudedinto the second section 10 through each polymer opening 20 at angleranging from about 20 degrees to about 160 degrees with respect to thetangent at the hot wall 6 in the second section 10 of the gas passage 4.

Referring to FIGS. 10-11, the first and second walls (5, 6, and 51, 61)of the gas passage 4 of these alternate configurations of nozzle 1 aresloped and planar and the transition region 9 is an asymmetric bentconfiguration located between the upstream section 8 and downstreamsection 10 where polymer 2 is introduced in this illustration.

The nozzle devices used to practice the processes described herein areconfigurable to be a cartridge that is mountable to conventional diebodies. Conventional die bodies may vary. However, industry standardmachine practice can be followed to mount the cartridge embodying theinventive nozzle to a die body. For example, the die bearing the nozzlecan be mounted to a die body with a conventional bolt arrangement andflat/shaped surfaces. If a gasket/seal is needed, the channel ismachined in the top of the die and/or the location is defined perspecific die body. For example, the nozzle system of the presentinvention may be adapted to fit the lower extrusion body of standardmeltspun equipment, e.g., equipment supplied by suppliers such asReifenhauser, Ason-Neumag, Lurgi Zimmer, Accurate Products, Nordson, andImpianti. Pressurized gas may be supplied to the nozzle system via gasmanifolds used in combination with extruder bodies in conventional orcommercial equipment or via another source of compressed gas fed to thenozzle inlet via air-tight fluid conduits and connections.

To implement the processes of the invention using the nozzle systems andsupport equipment illustrated above, the polymer generally is heateduntil it forms a liquid and flows easily. As indicated in the figures,the polymer melt is introduced into the second section 10 of the gaspassage 4 of the nozzle system 1 via opening 20, and forms a film as itdescends along the wall surface 6 where located below the opening 20,such as described above. To form the polymer melt, the polymer is heatedsufficiently to form a molten polymer flow. By way of example and notlimitation, the melted polymer may have a viscosity at the point offiberization in the nozzle as a positive numerical value less than 30Pa-s, particularly less than 20 Pa-s, and may range from 0.1 to 20 Pa-s,and particularly from 0.2 to 15. These viscosities are given over ashear rate ranging from about 100 to about 100,000 per second (at 240°C.). The melted polymer generally is at a pressure exceeding atmosphericpressure at the time it is conducted through the gap in the polymer dieand is introduced into the gas passage of the nozzle design.

The suitable and optimal melt flow rates of the starting polymermaterial used to provide the polymer melt may vary depending on the typeof polymer material used and other process conditions such as the gasflow properties. In the instance of polypropylene having a glasstransition temperature of approximately minus 18° C., a suitable meltflow rate may range, e.g., from about 35 to greater than 2000 decigramper minute, preferably not greater than 1800. The melt flow rate ismeasured using ASTM method D-1238. If the polymer material used ispolypropylene, it may have a polydispersity index (PDI) that ranges,e.g., from about 2.0 to about 4.0. For purposes herein, PDI is a measureof the distribution of molecular weights in a given polymer sample,where the PDI calculated is the weight average molecular weight dividedby the number average molecular weight.

The polymer throughput in the inventive process and apparatus willprimarily depend upon the specific polymer used, the nozzle design, andthe temperature and pressure of the polymer. The aggregate polymerthroughput of the nozzle system 1 is more than about 1 kg/hr/m,particularly may range from 1 to 200 kg/hr/m, more particularly from 10to 200 kg/hr/m, and most particularly between 25 and 70 kg/hr/m. Perorifice, the polymer throughput can be more than about 1, particularlymore than about 50, and more particularly more than about 1000 gram perminute per orifice. There can be several introduction gaps or orificesoperating at one time to increase the total production throughput. Thethroughput, along with pressure, temperature, and velocity, are measuredat the die orifice exit. A gas curtain or other ancillary gas stream canalso be used to affect the spray pattern of sub-micron fibers from twoor more nozzles. This gas stream or curtain may aid in shielding thespray formations between adjacent nozzles or may aid in compressing thespray pattern. The gas curtain or stream may improve the uniformity ofthe web.

The inventive nozzle system is not limited to any particular type ofpolymer material and allows for the polymer to be independently selectedfor a specific product application from amongst a wide variety ofpolymeric materials. Suitable polymeric materials for formation of thefibrous webs of the present invention are those polymers capable ofbeing fibrillated into microfibers using the nozzles of the presentinvention. These polymers include, but are not limited to polymersselected from the group consisting of polyolefins, polyesters,polyamides, biodegradable polymers, polyurethanes, polystyrenes, alkydresins, poly-hydroxyalkanoic acids, adhesives or other compounds capableof making fibers, and combinations thereof. Particular examples of thepolymeric materials are, e.g., polypropylenes. The polymers may befurther selected from homopolymers; copolymers, and conjugates and mayinclude those polymers having incorporated melt additives orsurface-active agents or pigments. More than one polymer type may beused at one time via the use of multiple polymer passages 200 a-d asillustrated in FIG. 3. In such embodiment, a web 13 comprisingmulticomponent sub-micron fibers may be produced, as described earlier.

The gaseous fluid may be introduced into the nozzle system at atemperature less than that of the polymer melt, and particularly below100° C., more particularly less than 50° C., or otherwise at roomtemperature (e.g., about 30° C., or less). The gaseous fluid also may beheated, although not required for processes of the present invention.Non-limiting examples of the fiberizing gaseous fluid are gases such asair, nitrogen, steam, etc. Additionally suitable gases may includereactive gases or gases with reactive components, or combinationsthereof. The pressure of the fiberizing (i.e., fibrillating) gaseousfluid is a positive pressure sufficient to blow the sub-micron fibersand can be slightly above the pressure of the melted polymer as it isextruded out of the gap from which it is introduced into the gas passageof the nozzle system. The fiberizing gaseous fluid will generally have apressure below 1000 psi, particularly will be less than 100 psi, moreparticularly will be from about 15 to about 80 psi. The gas flow rateused is sufficient to shear the polymer film at a sufficient rate tofibrillate. The gas flow rate through the nozzle system generally is inthe range of 150 kilogram per hour per meter to about 3500 kilogram perhour per meter, particularly 600 to 2000 kilogram per hour per meter;more particularly 1000 to 1800 kilogram per hour per meter. In terms ofgas mass flux, measured as unit mass of gas flowing per unit time perunit area, the gas flow ranges from about 15 kg/s/m² to about 1500kg/s/m² depending on the separation between the opposing walls 5 and 6in throat section 9 and gas flow rate used. For purposes herein, thecross-section of the transition region 9 of the nozzle 1 generally isused for the calculations of gas mass flux.

As one benefit of the present invention, increased sub-micron fiberoutput is obtained with lower gas demand, making it feasible to reduceenergy consumption and/or use smaller scale unit operations to stillprovide commercially significant sub-micron fiber output levels. In oneembodiment, the gas stream and polymer fluid stream are introduced intothe second section at a gas stream/polymer fluid stream mass flow rateratio generally less than about 40:1, particularly less than about 30:1;more particularly less than about 15:1. In one embodiment, the gasstream to polymer fluid stream mass flow ratio may be even less than10:1. The gas stream to polymer fluid stream mass flow ratio iscalculated as kilogram per hour per meter of gas stream through the gaspassage to kilogram per hour per meter of polymer fluid stream throughall the polymer openings in the second section. Equivalently, the gasstream to polymer fluid stream mass flux ratio is less than about 20:1,more preferably less than about 10:1, and most preferably less thanabout 7:1. The gas stream/polymer fluid stream mass flux ratio iscalculated as kg/s/m² of gas mass flux through the gas passage to thekg/s/m² of polymer fluid mass flux flowing through all the polymeropenings in the second section of the gas passage. Therefore, improvedperformance is provided via a more efficient, lower cost processdelivering higher quality microfiber or sub-micron fiber webs atcommercially viable outputs. Among other advantages, the process is moreefficient as it prevents polymer back flow and/or build-up on thegas-side in the fiber forming process. The resulting product web or matis high quality, as the web possesses good uniformity even at sub-micronfiber sizes, and with less fiber and web defects.

High quality microfibers are provided by the present invention withinnarrow fiber size distributions with minimal fiber defects. For purposesherein, a “high quality” fiber is defined as predominantly sub-micronfibers in a narrow fiber diameter distribution with minimal fiberdefects such as shot and dust. “Shot” is defined as unfiberizeddiscrete, largely spherical or ellipsoidal or combinations thereofpolymer mass with the largest dimension of the discrete mass rangingfrom 10 to 500 microns. By way of non-limiting illustration, FIG. 15 isa representative view (500×) showing microfibers with shot produced withstandard fiber making equipment and process conditions. The shot createsand leaves large pores and other defects in web formed by themicrofibers. FIG. 16 is a representative view (500×) showing microfiberswith very little or no shot made with a nozzle system operated accordingto an embodiment of the present invention. A good fiber distribution isprovided and more efficient polymer-fiber and web quality conversion isattained in the fibrous web shown in FIG. 16. “Dust” is another fiberdefect, where polymer is sheared uncontrolled or excessively to largelyspherical or ellipsoidal or combinations thereof polymer mass with thelargest dimension of less than 10 micron. An undesired, low quality offiber may have an excessive range of fiber diameters, or contain largeamount of dust or large amounts of shot. A particularly low quality offiber, or effect of fiberization, can contain large shot, typicallygreater than 40 micron in diameter, wherein the mass has enough momentumand thermal energy (i.e., temperature) to penetrate through the entirethickness of the nonwoven web to form a distinct “pin hole” defecttherein that can be visually identified well under magnified examination(i.e., over 10 times or 10×) of the web. Therefore, a web made from highquality fiberization, and fibers, has a narrow fiber diameterdistribution, no or only a low amount of dust below an average of 10particles per square-millimeter, no or a low amount of shot below anaverage of 10 particles per square-millimeter, and no or negligibleamount of penetrating and pin-hole creating type of shot. Thesemeasurements and evaluations can be done using optical microscopes withmagnification of 10× or preferably 100× (for shot), and with scanningelectron microscope photographs (for dust and shot). To determine anaverage, at least ten or preferably over twenty samples need to be takenfrom a production condition or a selected production period andevaluated in this manner for dust and shot.

The fibrous webs produced in accordance with the present invention mayinclude fibers exhibiting one or more fiber diameters. The fiberdiameters can range from sub-micron fiber diameters up to microfiberdiameters. For purposes herein “fiber diameter” is determined by SEMwith image analysis. Although not limited thereto, the average fiberdiameters may be, e.g., about 0.1 to about 1 micron, particularly about0.1 to about 0.9 micron, and more particularly about 0.3 to about 0.6micron. The raw nonwoven web product materials directly collected fromthe process of the present invention generally may comprise more than35%, particularly more than 75%, more particularly more than 95%, andmore particularly more than 99% sub-micron fibers. The standarddeviation of fiber diameter distribution generally may be less thanabout 0.5 micron, particularly less than about 0.3 micron. Further, thenonwoven fabric of the present invention may exhibit basis weightsranging from very light to very heavy. For example, and not by way oflimitation, the fabrics may have a basis weight ranging from less thanabout 5 grams per square meter (gsm), to fabrics having a basis weightgreater than about 200 gsm. In a particular embodiment, nonwoven productwebs comprising fibers in the indicated submicron fiber ranges have abasis weight of from about 0.01 to 200 gsm, particularly about 0.1 toabout 50 gsm. The basis weight of the nonwoven web products may bevaried depending on the web application envisioned. For some lighterweight applications, the basis weight of the sub-micron fiber layer maybe, for example, less than about 10 gsm, depending upon use of thenonwoven web. It may be desirable to form a web of several superposedlayers. The sub-micron fiber layer may be combined with one, two, ormore same or different layers. A composite web could comprise, forexample, a spunbond layer/sub-micron fiber layer/spunbond layerthree-component construction. Another example composite web could becomprised of a spunbond layer/1-10 micron fiber meltblownlayer/sub-micron melt-film-fibrillation fiber layer/spunbond layerconstruction. Basis weights for the total composite webs may range, forexample, from about 5 gsm to about 200 or more gsm, but may varydepending on the number and types of layers assembled together.

A uniform sub-micron fiber web can be produced by the process of thepresent invention. Web uniformity can be measured through severalmethods. In addition to the shot and dust rate described above, otherexamples of uniformity metrics include low coefficient of variation ofpore diameter, basis weight, air permeability, and/or opacity.Uniformity can also mean lack of fiber bundles or roping, or visibleholes, or other such defects. Uniformity may also be evaluated by thehydrohead or other liquid barrier measurement of the web. Pore diametercan be determined by methods known to those skilled in the art. The meanpore diameter of the sub-micron fiber layer may be less than about 15microns. The desired coefficient of variation for a uniform web can beless than 20%. The lack of roping can be measured by counting the numberof ropes or bundles of fibers in a measured area of the web; this isbest done jointly with a shot and dust evaluation. The lack of holes canalso be measured by counting the number of holes having a diameter abovea certain threshold in a measured area of the web. An optical microscopewith 10-100× magnification, or scanning electron microscope or otherenlargement means can be used. For example, the holes may be counted ifthey are visible to the naked eye using a light box, or are more than100 microns in diameter.

The present invention can be implemented on a wide variety polymermaterials and the nonwoven web may be used in a wide variety of articlesby itself or in combination with other materials. The nonwoven fabricproduced in accordance with the invention may include one or morefibrous layers, as well as wovens, scrims, films, and combinationsthereof, and may be utilized in the manufacture of numerous homecleaning, personal hygiene, medical, and other end use products where anonwoven fabric can be employed. The nonwoven web may be used, forexample, in gas or liquid filters, medical apparel, medical cleaningwipes, housewrap construction materials, diapers, training pants, adultincontinence pads, catamenials products such as feminine care pads andpantiliners, tampons, personal cleansing articles, personal carearticles, and personal care wipes such as baby wipes, facial wipes, bodywipes and feminine wipes, and combinations thereof. In addition, thefabric may be utilized as medical gauze, or similar absorbent surgicalmaterials, for absorbing wound exudates and assisting in the removal ofseepage from surgical sites. Other end uses include wet or dry hygienic,anti-microbial, or hard surface wipes for medical, industrial,automotive, home care, food service, and graphic arts markets, which canbe readily hand-held for cleaning and the like.

The nonwoven of the present invention also may be included in constructssuitable for medical and industrial protective apparel, such as gowns,drapes, shirts, bottom weights, lab coats, face masks, and the like, andprotective covers, including covers for vehicles such as cars, trucks,boats, airplanes, motorcycles, bicycles, golf carts, as well as coversfor equipment often left outdoors like grills, yard and gardenequipment, such as mowers and roto-tillers, lawn furniture, floorcoverings, table cloths, and picnic area covers. In particularembodiment, the nonwoven is used in an article selected from the groupconsisting of bandages, diapers, training pants, adult incontinencepads, catamenials products such as feminine care pads and pantiliners,tampons, personal cleansing articles, personal care articles, andpersonal care wipes such as baby wipes, facial wipes, body wipes andfeminine wipes, and combinations thereof. The nonwoven may also be usedin top of bed applications, including mattress protectors, comforters,quilts, duvet covers, and bedspreads. Additionally, acousticalapplications, such as interior and exterior automotive components,carpet backing, insulative and sound dampening appliance and machinerywraps, and wall coverings. The nonwoven is further advantageous forvarious filtration applications, including bag house, plus pool and spafilters. The nonwoven also may be used in other applications, such asbattery separators, or as agent/particle carriers (e.g., catalystcarriers).

Depending on the desired end use application of the nonwoven fabric,specific additives may be included directly into the polymeric melt orapplied after formation of the web. Suitable non-limiting examples ofsuch additives include absorbency enhancing or deterring additives, UVstabilizers, fire retardants, dyes and pigments, fragrances, skinprotectant, surfactants, aqueous or non-aqueous functional industrialsolvents such as, plant oils, animal oils, terpenoids, silicon oils,mineral oils, white mineral oils, paraffinic solvents, polybutylenes,polyisobutylenes, polyalphaolefins, and mixtures thereof, toluenes,sequestering agents, corrosion inhibitors, abrasives, petroleumdistillates, degreasers, and the combinations thereof. Additionaladditives include antimicrobial composition, including, but not limitedto iodines, alcohols, such as such as ethanol or propanol, biocides,abrasives, metallic materials, such as metal oxide, metal salt, metalcomplex, metal alloy or mixtures thereof, bacteriostatic complexes,bactericidal complexes, and the combinations thereof.

All amounts, parts, ratios, and percentages described herein are byweight unless otherwise indicated. The following non-limiting examplefurther illustrates the present invention.

EXAMPLE

An extruder (2.5 inch diameter, single screw extruder) and aconventional melt blown die body (25 inch width) were used to provide asource of 1800 MFR polypropylene. The extruder temperature was 650° F. Anozzle generally having the configuration of FIG. 4 was mounted to aconventional extruder die body using a conventional gasketed bolt mountat an upper flat surface region on the nozzle device. A source ofpressurized air was fed from an air supply to the inlet of the nozzlevia air-tight connections and seals. The nozzle had the followinggeometrical features (using FIG. 4 as a non-limiting example): an 0.016inch as the minimum distance between opposing walls 5 and 6 in thethroat section 9; cold wall 5 converging at an angle θ of negative 1.5degrees towards the bisecting plane 7; hot wall diverging away at anangle β of 2 degrees from the bisecting plane 7; the polymer passageentered the second section in the second, downstream half of the secondsection and had a hydraulic diameter of about 0.008 inch and wasoriented at an angle γ of about 32 degrees with the hot wall 6; thepolymer fiberization surface length δ was almost zero. The convergingsection 8 had a vertical length of about 0.090 inch with the bisectionangle α about 30 degrees. The throat section 9 had a vertical length ofabout 0.010 inch, and the diverging section 10 had a vertical length ofabout 0.200 inch. The pressurized air was introduced into the inlet end(converging section) of the nozzle at a flow rate of 300 scfm (standardcubic feet per minute) and at an air temperature of 80° F. A nonwovenweb product was collected and analyzed which revealed that it had thefollowing product attributes: 17.2 gsm basis weight total of thespunbond-layer/submicron-fiber layer/spunbond-layer construction;estimated fiber content produced from the nozzle apparatus of currentinvention was about 15% (2.7 gsm); Mean diameter of the fibers in thesubmicron-fiber layer: 0.45 micron; standard deviation: 0.15; ratio ofstandard deviation/mean of submicron fiber diameter distribution=0.33;and fiber diameter range: 0.1 to 0.85 microns.

From the foregoing, it will be observed that numerous modifications andvariations can be affected without departing from the true spirit andscope of the novel concept of the present invention. It is to beunderstood that no limitation with respect to the specific embodimentsillustrated herein is intended or should be inferred. The disclosure isintended to cover, by the appended claims, all such modifications asfall within the scope of the claims.

1. A process for making a nonwoven web, comprising: a pressurized gasstream which flows within a gas passage that is confined between firstand second opposing walls, wherein at least one of the opposing walls isheated; said gas passage comprising a first, upstream section into whichthe gas enters from a supply end, a transition region, and a second,downstream section in which the gas flows to an exit end, wherein thetransition region fluidly connects the first section to the secondsection, and the gas passage ends at the exit end of the second section,and wherein said first section of the gas passage having a decreasingcross-sectional area from the supply end to the transition region, andsaid second section of the gas passage having an increasingcross-sectional area from the transition region to the exit end of thesecond section; at least one polymer fluid stream flowing through atleast one bounded polymer passage which ends in at least one opening inat least one of said opposing walls that is heated, wherein each polymerfluid stream extrudes in the form of a film from each said opening toprovide an extruded polymer film on a heated wall surface; and eachextruded polymer film joining with the gas stream and the polymer filmbeing fibrillated to form fibers comprising sub-micron diameter fibersexiting from the exit end of said second section of the gas passage. 2.The process of claim 1, wherein the gas stream is introduced into thegas passage at a mass flow rate ranging from about 150 kilogram per hourper meter to about 3500 kilogram per hour per meter.
 3. The process ofclaim 1, wherein gas stream and polymer fluid stream are introduced at agas stream/polymer fluid stream mass flow rate ratio less than about40:1.
 4. The process of claim 1, wherein the gas stream at the entry ofthe gas passage is at a temperature less than 50° C.
 5. The process ofclaim 1, wherein the nonwoven web material comprises more than 35%sub-micron diameter fibers.
 6. The process of claim 1, wherein thestandard deviation of fiber diameter distribution of said fibers is lessthan about 0.5 micron.
 7. The process of claim 1, wherein the standarddeviation of fiber diameter distribution of said fibers is less thanabout 0.3 micron.
 8. The process of claim 1, wherein the fibers arecomprised of a polymer selected from the group consisting ofpolyolefins, polyesters, polyamides, biodegradable polymers,polyurethanes, polystyrenes, alkyd resins, poly-hydroxyalkanoic acids,and combinations thereof.
 9. A process for making a nonwoven web,comprising: a pressurized gas stream which flows within a gas passagethat is confined between first and second opposing walls, wherein atleast one of the opposing walls is heated; said gas passage comprising afirst, upstream section into which the gas enters from a supply end, atransition region, and a second, downstream section in which the gasflows to an exit end, wherein the transition region fluidly connects thefirst section to the second section, and the gas passage ends at theexit end of the second section, and wherein said first section of thegas passage having a monotonically decreasing cross-sectional area fromthe supply end to the transition region, and said second section of thegas passage having a monotonically increasing cross-sectional area fromthe transition region to the exit end of the second section; at leastone polymer fluid stream flowing through at least one bounded polymerpassage which ends in at least one opening in at least one of saidopposing walls that is heated, wherein each polymer fluid streamextrudes in the form of a film from each said opening to provide anextruded polymer film on a heated wall surface; and each extrudedpolymer film joining with the gas stream in the second section of thegas passage, and the polymer film being fibrillated to form fiberscomprising sub-micron fibers exiting from the exit end of said secondsection of the gas passage.
 10. The process of claim 9, wherein eachpolymer passage opening is a slit with a hydraulic diameter defined asfour times cross-sectional area of the polymer passage opening dividedby inner perimeter of the polymer passage opening, said hydraulicdiameter of each polymer passage opening ranging from about 0.001 inchto about 0.100 inch.
 11. The process of claim 10, wherein a firstbisecting surface, defined as an angular bisector of the angle betweenthe first and second walls, geometrically divides the first section intotwo halves with about equal volumes, and a second bisecting surface,defined as an angular bisector of the angle between the first and secondwalls, geometrically divides the second section into two halves withabout equal volumes.
 12. The process of claim 11, wherein the bisectionangle of the first and the second walls with respect to the firstbisecting surface ranges from about 15 to about 40 degrees in the firstsection, and where the bisection angle of the first and the second wallswith respect to the second bisecting surface ranges from about 2 toabout 20 degrees in the second section of the gas passage.
 13. Theprocess of claim 12, wherein the first and second walls of the gaspassage are heated to about a same temperature to provide symmetricthermal states with respect to the first and the second bisectingsurfaces.
 14. The process of claim 13, wherein only one polymer filmforms on at least one of the opposite heated walls, and each polymerpassage opening from which polymer film extrudes is located in the upperhalf of the second section as determined relative to the length of thepolymer passage-including heated wall that extends between thetransition region and the exit end of the second section.
 15. Theprocess of claim 14, wherein the first and second walls of the gaspassage are smoothly curved such that the curvature of the opposingwalls in the first section smoothly transitions without any sharp edgesinto the curvature of the opposing walls in the second section in thetransition region of the gas passage.
 16. The process of claim 10, whereone of the opposing walls is a hot wall and the other wall is a coldwall, wherein temperature of the hot wall is at least 50° C. higher thanthe cold wall, and only the hot wall has at least one polymer fluidpassage opening.
 17. The process of claim 16, wherein the bisectionangle of the first and the second walls with respect to the firstbisecting surface in the first section ranges from about 15 to about 40degrees.
 18. The process of claim 17, where the hot wall in the secondsection diverges away from the first bisecting surface at an angle thatranges from about 1 degree to 20 degrees, and the cold wall in thesecond section converges towards the first bisecting surface at an anglethat ranges from about 0.1 degree to about 15 degrees.
 19. The processof claim 9, wherein the angle between centerline of each polymer passageand the wall containing the corresponding polymer passage opening rangesfrom about 10 degrees to about 100 degrees.
 20. The process of claim 19,wherein the polymer film extruding from each polymer passage openingflows with the gas flow along a polymer fiberization surface, which hasan orientation angle with respect to the first bisecting surface rangingfrom about 90 degrees measured in clockwise direction to about 45degrees measured in counterclockwise direction.
 21. The process of claim20, wherein length of the polymer fiberization surface corresponding toeach polymer passage opening is less than about one thousand times thehydraulic diameter of the corresponding polymer passage opening.
 22. Theprocess of claim 18, wherein the ratio between the diverging angle ofthe hot wall relative to the first bisecting surface and the convergingangle of the cold wall relative to the first bisecting surface rangesfrom about 1:1 to about 500:1.
 23. The process of claim 21, wherein thefirst and second walls of the gas passage are smoothly curved such thatthe curvature of the opposing walls in the first section smoothlytransitions without any sharp edges into the curvature of the opposingwalls in the second section in the transition region of the gas passage.24. The process of claim 23, wherein the opposing walls in the secondsection of the gas passage are curved such that the hot wall has aconvex shape that curves away from the second bisecting surface and thecold wall has a concave shape that curves towards the second bisectingsurface as viewed from within the second section in the gas passage. 25.The process of claim 18, wherein the ratio of the radius of curvature ofthe hot wall to the radius of curvature of the cold wall in the secondsection of the gas passage ranges from about 1:10,000 to about 100:1.26. The process of claim 10, wherein the nonwoven web comprises morethan 99% fibers with diameter less than about 1 micron.
 27. The processof claim 10, wherein the gas stream is introduced into the gas passageat a mass flow rate ranging from about 150 kilogram per hour per meterto about 3500 kilogram per hour per meter.
 28. The process of claim 10,wherein gas stream and polymer fluid stream are introduced into thesecond section at a gas stream/polymer fluid stream mass flow rate ratioless than about 40:1.
 29. The process of claim 10, wherein the gasstream at the entry of the gas passage is at a temperature less than 50°C.
 30. The process of claim 10, wherein the nonwoven web materialcomprises more than 35% sub-micron fibers.
 31. The process of claim 10,further comprising providing a plurality of different polymer materialsas different polymer fluid streams flowing through separate boundedpolymer passages which end in an opening in at least one of saidopposing heated walls, wherein the different polymer fluid streamsextrude in the form of a films from each said opening, wherein thenonwoven web material comprises multi-component fibers.
 32. The processof claim 1, further comprising impinging the extruded polymer film onthe heated wall surface by the gas stream flowing within the gaspassage, effective to fibrillate the polymer film into the sub-microndiameter fibers.
 33. The process of claim 9, further comprisingimpinging the extruded polymer film on the heated wall surface by thegas stream flowing within the gas passage, effective to fibrillate thepolymer film into the sub-micron diameter fibers.